The present invention relates to a thermal recording material, which has good color developing sensitivity, excellent degree of whiteness of ground color, good image retention and strong resistance to heat.
In general, a thermal recording material having a thermally sensitive color developing layer mainly composed of colorless or pale colored basic colorless dye and color developing agent that develops color by reacting with said basic colorless dye when heated is disclosed in Japanese patent publication 45-14039 and widely used in commercial scale. A thermal printer in which a thermal head is installed is used for the recording method of said thermal recording material. Since this kind of thermal recording method superiors to the conventional recording method from the view point of noiseless at recording process, does not need developing and fixing processes, maintenance free, equipment is relatively cheap and compact and the obtained image is very clear, therefore, this method is widely applied in the field of facsimile or computer, various kinds of measuring instrument and for a labeling machine along with the growth of an information industry. The recording devices attached to these instruments are becoming more diversified and more high-performance, and the required quality to the thermal recording material is becoming more severe. Along with the miniaturization of an instrument and requirement for higher recording speed, it becomes necessary to obtain deep and clear color developing image. Further, more close qualities to ordinary paper are required to the thermally sensitive recording paper. Namely, for instance, to have an excellent whiteness, the coloring of ground color is very small at high temperature and high humidity condition and the fading of color developed part is very small.
For the purpose to meet said requirement, for example, in the Japanese Patent Laid open Publication 56-169087, a method to improve the color developing sensitivity by adding a thermo fusional substance to a thermally sensitive layer is disclosed, and in the Japanese Patent Laid Open Publication 56-144193, a method to improve the color developing sensitivity by using new color developing agent which has high color developing ability is disclosed. However, these methods can not be said to have a sufficient quality, because these methods have defects such as deterioration of ground color by heat, powder generation by aging and dropping of color density after preserved for long time (reprinting ability).
As the compound that is usually used as the color developing agent, sulfone type compound or salitylic acid type compound can be mentioned besides the phenol type compound. However, not only the resistance to heat or to humidity of these compounds of themselves is not sufficient, but also when used together with a sensitizer, the resistance to heat or to humidity is further deteriorated. Dihydroxydiphenylsulfone compound which is used as an organic color developing agent in this invention is a well-known compound, however, the color developing ability of it is low and it is difficult to meet the current requirement for high sensitivity. Further, when a coating for the thermally sensitive recording layer is prepared, the coloring problem of coating itself occurs, therefore, it is difficult to obtain the thermal recording material that is excel in degree of whiteness of the ground color.
The object of this invention is to provide a thermal recording material which has high color developing ability and excellent degree of whiteness of the ground color, further, excellent image retention and strong resistance to heat.
The inventors of this invention have conduced an intensive study and have found that in a thermal recording material that possesses a thermally sensitive recording layer containing colorless or pale colored basic colorless dye and organic color developing agent as a main component, when said thermally sensitive recording layer contains at least one kind of dihydroxydiphenylsulfone type compound represented by following general formula (1) and contains 3-di-n-pentylamino-6-methyl-7-anilinofluoran as a basic colorless dye, the above mentioned object can be achieved, and accomplished the present invention. 
In the formula, R1 and R2 indicate alkyl group or alkenyl group of carbon number 1 to 8, or halogen atom and p and q indicate an integer number of 0 to 3.
In general, it is concerned that the coloring of coating is caused by partially dissolving of the material contained in the coating in water and the dissolved material reacts with dye and cause the coloring. For example, the degree of whiteness of thermal recording material that contains 4,4xe2x80x2-dihydroxydiphenylsulfone is remarkably deteriorated. The reason why is concerned that because 4,4xe2x80x2-dihydroxydiphenylsulfone contains two xe2x80x94OH groups, which make the basic colorless dye develop color and is easily soluble in water and the polarity of the molecular is comparatively high. Further, when basic colorless dye is more finely ground, the deterioration of degree of whiteness becomes more remarkable. By the intensive study of the inventors, it is found that the combination use of 3-di-n-pentylamino-6-methyl-7-anilinofluoran as the basic colorless dye can specifically prevent the coloring of the coating and prevent the deterioration of whiteness of thermal recording material even if the basic colorless dye is finely ground. The reason why is not made clear, however, it is guessed that because the solubility of 3-di-n-pentylamino-6-methyl-7-anilinofluoran is smaller than 1.349xc3x9710xe2x88x926 g/l, which is concerned very low solubility.
The thermal sensitive recording medium of this invention, for example, can be produced by following method. That is, prepare the dispersion in which basic colorless dye, color developing agent are dispersed with a binder, then a filler and other necessary additives are added and mixed. Thus the coating for a thermal sensitive color developing layer is prepared. The prepared coating is coated on a substrate and dried up so as to form a thermal sensitive color developing layer. The desirable mixing ratio of basic colorless dye and color developing agent is 1-6 parts of color developing agent to 1 part of dye, and the mixing ratio is in said range, the color developing sensitivity and the ground color resistance to heat are well balanced.
In this invention, dihydroxydiphenylsulfone compound represented by general formula (1) is contained as an organic color developing agent. The concrete example of general formula (1) can be mentioned below. However, not intend to be limited to these compounds. And these compounds can be used alone or together with.
4,4xe2x80x2-dihydroxydiphenylsulfone,
2,4xe2x80x2-dihydroxydiphenylsulfone,
bis-(3-allyl-4-hydroxyphenyl)sulfone,
3,3xe2x80x2-dimethyl-4,4xe2x80x2-dihydroxydiphenylsulfone,
3,3xe2x80x2,5,5xe2x80x2-tetramethyl-4,4xe2x80x2-dihydroxydiphenylsulfone,
2,2xe2x80x2-bis(4-chlorophenol)sulfone,
4-hydroxyphenyl-3xe2x80x2-isopropyl-4xe2x80x2-hydroxyphenylsulfone,
bis-(3-ethyl-4-hydroxyphenyl)sulfone,
2,2xe2x80x2-bis(p-t-butylphenol)sulfone,
2,2xe2x80x2-bis(p-t-pentylphenol)sulfone and
2,2xe2x80x2-bis(p-t-octylphenol)sulfone
Among these compounds, 4,4xe2x80x2-dihydroxydiphenylsulfone is preferably used, because of the qualities of obtained thermal recording material are well balanced, and especially the image retention in high temperature and in high humid condition are excellent.
In this invention, the average particle size of basic colorless dye and color developing agent are not restricted, however, when the average particle size of 3-di-n-pentylamino-6-methyl-7-anilinofluoran, which is basic colorless dye is smaller than 0.5 xcexcm, the color developing sensitivity and the resistance to heat is improved. More desirable average particle size is smaller than 0.4 xcexcm. In general, when the particle size of dye becomes smaller, it becomes more easily soluble in water and the reactivity between dye and color developing agent is improved because these compounds exist in molecular condition, the problem of ground color coloring is caused. However, in a case of 3-di-n-pentylamino-6-methyl-7-anilinofluoran, such kind of problem does not cause, and the thermal recording material of high quality can be obtained. The reason why is not made clear, however, it is concerned because the solubility to water is very poor.
In this invention, conventional well known sensitizer can be used in the limitation not to prevent the desired effect. As the concrete examples of the sensitizer, compounds mentioned below can be used, however, not intend to be limited to these compounds. And these compounds can be used alone or
together with,
fatty acid amide,
ethylene bis amide,
montanic acid wax,
polyethylene wax,
1,2-di-(3-methylphenoxy)ethane,
diphenylsulfone,
p-benzylbiphenyl,
xcex2-benzyloxynaphthalene,
4-biphenyl-p-tolylether,
m-tarphenyl,
1,2-diphenoxyethane,
dibenzyl 4,4xe2x80x2-ethylenedioxy-bis-benzoate,
dibenzoyloxymethane,
1,2-di-(3-methylphenoxy)ethylene,
1,2-diphenoxyethylene,
bis[2-(4-methoxy-phenoxy)ethyl]ether,
p-nitromethylbenzoate,
dibenzyloxalate,
di(p-chlorobenzyl)oxalate,
benzyl p-benzyloxy benzoate,
di-p-tolylcarbonate,
phenyl-xcex1-naphthylcarbonate,
1,4-diethoxynaphthalene,
phenyl 1-hydroxy-2-naphthoate,
o-xylene-bis-(phenylether)
4-(m-methylphenoxymethyl)biphenyl
o-toluensulfoneamid and
p-toluensulfoneamid
can be mentioned, however, not intends to be limited to them. These sensitizer can be used alone or can be used together with.
By the use of a sensitizer, sometimes the resistance to heat or humidity is remarkably deteriorated. Therefore, basically the use of sensitizer is not desirable, and if it is necessary to be used, the kind of sensitizer must be carefully selected.
As the binder to be used in this invention,
full saponificated polyvinyl alcohol whose degree of polymerization is 200-1900,
partially saponificated polyvinyl alcohol,
carboxyl denatured polyvinyl alcohol,
amide denatured polyvinyl alcohol,
sulfonic acid denatured polyvinyl alcohol,
butyral denatured polyvinyl alcohol,
other denatured polyvinyl alcohol,
hydroxyethyl cellulose,
methyl cellulose,
carboxymethyl cellulose,
ethyl cellulose,
cellulose derivative such as acetyl cellulose,
polyvinyl chloride,
polyvinyl acetate,
polyacrylamide,
polyacrylamideester,
polyvinyl butyral,
polystyrol and these copolymer,
styrene-maleic acid unhydride copolyme,r
styrene-buthadiene copolymer,
silicone resin,
petroleum resin,
terpene resin,
ketone resin and
coumarone resin
can be mentioned. These high polymer substances can be used by dissolving in solvents such as water, alcohol, ketones, esters and hydrocarbons, or, can be used in a state of aqueous emulsion or paste condition at need.
Further, in this invention, as the image stabilizer showing oil resistance effect of recorded image,
4,4xe2x80x2-buthylidene(6-t-buthyl-3-methylphenol),
2,2xe2x80x2-di-t-buthyl-5,5xe2x80x2-dimethyl-4,4xe2x80x2-sulphonyidiphenol,
1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)buthane,
1,1,3-tris(2-methyl-4-hydroxy-5-t-buthylphenyl)buthane,
4-benzyloxy-4xe2x80x2-(2,3-epoxy-2-methylpropoxy)diphenylsulfone and
epoxy resin can be added in the limit not to prevent above mentioned desired effect.
As a filler which can be used in this invention, an inorganic or an organic filler such as silica, calcium carbonate, kaoline, calcined kaoline, diatomaceous earth, talc, titanium oxide or aluminum hydroxide can be mentioned. Further, a slipping agent such as waxes, an ultra violet ray absorbing agent such as benzophenone type or triasol type compound, a water proof agent such as glioxasal, a dispersing agent, a defoamer, an anti oxidation agent and fluoresene dye can be used.
Referring to the amount of color developing agent and basic colorless dye to be used to the thermal recording material of this invention, the kind and amount of them and other additives are decided according to the required quality and recording feature, and not restricted. However, in general, it is preferable to use 0.5-4 parts of filler to 1 part of color developing agent of this invention. And the desirable amount of binder is 5-25% to the total amount of solid.
The coating composed of above mentioned component is coated over the surface of substrate such as paper, recycled paper, synthetic paper, film, plastic film, plastic foam film or non-woven cloth, and the desired thermal recording material can be obtained. The complex sheet composed of above mentioned materials can also be used.
And, for the purpose to improve the preservative property, an overcoat layer composed of high polymer compound can be prepared on the thermally sensitive color developing layer. Further, for the purpose to improve the color developing sensitivity, an undercoat layer composed of high polymer substance containing a filler can be prepared under the thermally sensitive layer. The aforementioned organic color developing agent, basic colorless dye and other additives, which are added at need, are ground to the fine particles smaller than several microns diameter by means of a grinder or an adequate emulsifying apparatus, then substantially used. The method for pulverizing is not restricted, however, usually, materials such as basic colorless dye and color developing agent are dispersed with water and a binder to an uniform suspension, then ground using a ball mill, an attriter, a vertical sand grinder or a horizontal sand grinder.
Further, a binder and other additives are added at need and the coating is prepared. The method to coat is not restricted, however, can be coated by conventional well known methods, for example, an off machine coating machine with various coater such as air knife coater, rod blade coater, bill blade coater or roll coater, or an on machine coating machine can preferably be used.